The present invention relates to a bias circuit and a radio communication apparatus using the same, and particularly to a bias circuit for supplying a bias voltage to a gate of a FET and a radio communication apparatus such as a cellular telephone or a cordless telephone using the same as a gate bias circuit for an RF amplifying FET in a transmission system thereof.
In a radio frequency (RF) band, especially a microwave band, an N-channel FET having excellent RF characteristics, such as a GaAs FET, is used. In a circuit using the FET, a conventional bias circuit for providing a bias voltage to a gate of the FET is generally configured to use a divided voltage obtained by dividing power supply voltage by resistance as a gate bias voltage for the FET.
The gate bias voltage of the thus configured bias circuit for the FET is uniquely determined by the voltage dividing resistances. On the other hand, radio frequency FETs such as GaAs FETs greatly vary in threshold voltage Vth between production lots, and therefore the resistance division system in which the gate bias voltage is uniquely determined produces a great variation in idle current Idsq.
Thus, because of the variation in idle current Idsq, the conventional bias circuit employing the resistance division system cannot deliver specified performance, thereby requiring adjustment such as changing the voltage dividing resistances. Moreover, since the bias voltage of the conventional bias circuit obtained by the resistance division system is fixed, a variation in characteristics of the FET due to temperature variation or the like causes the idle current Idsq to vary according to the variation in characteristics of the FET.